How do I stop vertices being hidden by the Subsurf modifier in Edit mode?
There are times that the unmodified vertices of a mesh will be inside the surface created by a Subsurf modifier. 

Personally, I regularly flip between view subsurf levels while I'm modeling, anyway. (Ctrl0to5 on the keyboard) Then you can edit the model in its unmodified appearance, which is often less confusing than another option, (illustrated)



which is to 'Adjust edit cage to modifier resullt'. this will result in the appearance of sliding points around on the curved, subdivided surface.